Au, Pt, or Ag particles with particle sizes of ca. 1 μm were used as catalysts for boring pores in p-type Si(100) wafers by wet etching in aqueous solutions containing hydrofluoric acid and hydrogen peroxide. Boring speed was fastest when Pt particles were used as the catalyst. However, the sidewalls of the pores and the surface of the wafer were covered with a nanoporous silicon layer of ca. 500 nm in thickness, and the pore showed a tapered structure. When micrometre-sized Ag particles were used, no deep pores were formed because the particles were unstable in the solution. In contrast to Pt and Ag particles, Au particles bored straight pores under some conditions. However, the morphology of pores depended on the shape of the Au particles. Spherical Au particles formed straight pores, whereas non-spherical Au particles formed pores with spiral sidewalls. When Au particles formed aggregates consisting of a small number of particles (<10 particles), crooked pores tended to be formed. In contrast, when the aggregates were composed of a larger number of particles, straight pores were formed and the boring speed was faster than the pores formed with isolated Au particles.

Pore formation in silicon by wet etching using micrometre-sized metal particles as catalysts

Self-assembled monolayers (SAMs) of octyltrimethoxysilane (OTMS) were deposited onto indium−tin oxide (ITO) electrode surfaces. Contact angle measurements evidenced the hydrophobicity and homogeneity of the functionalized surface despite the intrinsic surface roughness of the polycrystalline ITO electrodes. The electrochemical properties of the OTMS monolayers were quantitatively analyzed in terms of resistance, dielectric thickness, diffusion constant, and defect area by cyclic voltammetry and impedance spectroscopy. It has been demonstrated that the alkylsiloxane monolayer acts as a diffusion barrier for ions in the electrolyte. Furthermore, a significant suppression of the charge transfer at the interface was observed, demonstrating the passivation effect of the monolayers against electrochemistry. In addition, the effect of alkyl chain length on the electrochemical properties was briefly analyzed by using octadecyltrimethoxysilane (ODTMS). The defect area of this self-assembled monolayer was reduced t...

Electrochemical Characterization of Self-Assembled Alkylsiloxane Monolayers on Indium−Tin Oxide (ITO) Semiconductor Electrodes

Vertically aligned silicon nanowire (SiNW) arrays have been fabricated over a large area using a silver-assisted single-step electroless wet chemical etching (EWCE) method, which involves the etching of silicon wafers in aqueous hydrofluoric acid (HF) and silver nitrate (AgNO3) solution. A comprehensive systematic investigation on the influence of different parameters, such as the etching time (up to 15 h), solution temperature (10–80 °C), AgNO3 (5–200 mM) and HF (2–22 M) concentrations, and properties of the multi-crystalline silicon (mc-Si) wafers, is presented to establish a relationship of these parameters with the SiNW morphology. A linear dependence of the NW length on the etch time is obtained even at higher temperature (10–50 °C). The activation energy for the formation of SiNWs on Si(100) has been found to be equal to ~0.51 eV . It has been shown for the first time that the surface area of the Si wafer exposed to the etching solution is an important parameter in determining the etching kinetics in the single-step process. Our results establish that single-step EWCE offers a wide range of parameters by means of which high quality vertical SiNWs can be produced in a very simple and controlled manner. A mechanism for explaining the influence of various parameters on the evolution of the NW structure is discussed. Furthermore, the SiNW arrays have extremely low reflectance (as low as <3% for Si(100) NWs and <12% for mc-Si NWs) compared to ~35% for the polished surface in the 350–1000 nm wavelength range. The remarkably low reflection surface of SiNW arrays has great potential for use as an effective light absorber material in novel photovoltaic architectures, and other optoelectronic and photonic devices.

https://iopscience.iop.org/article/10.1088/0957-4484/25/17/175601/pdf

Large area fabrication of vertical silicon nanowire arrays by silver-assisted single-step chemical etching and their formation kinetics.

Analysis of experimental constraints and variables for time resolved detection of Fourier transform electrochemical impedance spectra

Stable and high-performance nanoporous “black silicon” photoelectrodes with electrolessly deposited Pt nanoparticle (NP) catalysts are made with two metal-assisted etching steps. Doubly etched samples exhibit an ∼300 mV positive shift in photocurrent onset for photoelectrochemical proton reduction compared to oxide-free planar Si with identical catalysts. We find that the photocurrent onset voltage of black Si photocathodes prepared from single-crystal planar Si wafers by an Ag-assisted etching process increases in oxidative environments (e.g., aqueous electrolyte) owing to a positive flat-band potential shift caused by surface oxidation. However, within 24 h, the surface oxide layer becomes a kinetic barrier to interfacial charge transfer that inhibits proton reduction. To mitigate this issue, we developed a novel second Pt-assisted etch process that buries the Pt NPs deep into the nanoporous Si surface. This second etch shifts the onset voltage positively, from +0.25 V to +0.4 V versus reversible hydrog...

S. Ottow

Papers

Determination of flat-band potentials of silicon electrodes in HF by means of ac resistance measurements

In situ impedance spectroscopy of silicon electrodes during the first stages of porous silicon formation